Terbium activated luminescent glass and fiber optic luminescent glass x-ray detection plates are promising alternatives to polycrystalline phosphor screens in high energy x-ray (&gt;100 kV) electronic radiographic systems. These devices have recently been shown to provide images with improved spatial resolution and enhanced contrast sensitivity in these applications compared to the polycrystalline phosphor screens (see L. M. Klynn, and R. C. Barry, et al. "High Resolution Real-Time Radiography System Design" final report AF Contract F33615-83-C-5087, AFWAL-TR-87-4055, Jul. 1987).
The important characteristics to optimize in luminescent glass design to obtain further improvements in these systems are (1) a high x-ray absorption efficiency and (2) a high conversion of this absorbed energy into light for capture by a suitable light detector or camera.
To obtain high x-ray absorption efficiency in the 50 keV to 15 MeV energy regime typically employed for the purpose of nondestructive testing, the glass must have high density, a high effective atomic number (Z), and optimum thickness. To achieve a high x-ray-to-light conversion efficiency the terbium must provide a strong luminescence in the host employed. In some cases, due to poor host/activator interaction, making the glasses heavy to absorb x-rays reduces the x-ray-to-light conversion efficiency of the material. This has been observed for PbO, Sb.sub.2 O.sub.3, Fe.sub.2 O.sub.3, ZnO, As.sub.2 O.sub.3, HfO.sub.2, Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 addition to terbium activated luminescent glass (U.S. Pat. No. 3,654,172, "Terbium Activated Radioluminescent Silicate Glass", Apr. 4, 1972). Conversely, low Z glasses have shown high x-ray-to-light conversion, but poor x-ray absorption characteristics. For example, a luminescent terbium-activated lithium beryllium borate glass has been suggested for use under x-ray excitation in Ger. Offen. DE 2500910, Dec. 9, 1976. Obtaining both is the subject of this application.
Because of their high ultraviolet quantum yields, many terbium activated borate luminescent glass compositions have been developed for use under ultraviolet excitation in fluorescent lamps. Some have moderately high Z and density and it could be expected that these materials would be useful x-ray detection materials. However, these materials frequently do not perform well under x-rays. There are subtle energy transfer processes occurring under x-rays that do not occur under ultraviolet excitation. As x-ray energy is deposited into these materials, electrons and holes are generated throughout the host glass matrix. The incorporation of ions into the glass that alter the electronic nature of the host matrix can result in trapping of this energy or reduced transport of this energy. Both processes will inhibit the excitation of the activator ions. Under ultraviolet radiation, the activator can directly capture the ultraviolet energy. This reduces the interaction between the host glass ions and the activator ions. For these reasons, luminescent glasses that are useful for applications that employ ultraviolet excitation processes, will often not be useful for application under x-rays. Even co-activator (sensitizer) ions that are included into glass compositions to improve the response of the primary activator under uv excitation may quench the response this glass has under x-ray excitation.
Illustratively, in European Patent 338934, dated Oct. 25, 1989, and French Patent 2630430 dated Oct. 27, 1989, The inventors report that a terbium activated rare-earth borate glass sensitized with MnO has a high white luminescent response under ultraviolet radiation. However, we have found this material to have a rather weak green luminescent response under x-rays. In JP 58/69740 A2 [83/69740], dated Apr. 26, 1983, a Eu.sup.3+ sensitized terbium activated rare-earth borate glass has a response that is similarly quenched under x-ray excitation. This patent, however, covers a very broad range of constituents with no particular utility. We have found that when we tried to prepare glass materials from some of their compositions with gadolinium oxide concentrations between 15 and 20 mole %, these materials devitrified after removal from the hot zone and casting, and glasses could not be formed.
At the current state of knowledge in the design of x-ray to light conversion screens for radiographic applications, it is still not possible to predict with assurance which constituents will improve or hinder the x-ray response of a material.